Are you calling the super method that takes the RGB values in your model? A lot of people do this:
@Override
public void render(MatrixStack matrixStack, IVertexBuilder buffer, int packedLight, int packedOverlay, float red, float green, float blue, float alpha)
{
Model.render(matrixStack, buffer, packedLight, packedOverlay);
}
Note that the method takes in RGB and alpha, but calls a render method that doesn't take those values in, if that doesn't work you can always replace the vanilla rendering logic with your own, take a look at LivingRenderer if you're interested.